The present invention relates to a cylinder system installed in a suspension system of a vehicle, for example, an automobile. The present invention also relates to a vehicle height adjusting apparatus using such a cylinder system.
Generally, in automobiles or other vehicles, a hydraulic damper (cylinder system) is installed between a sprung member and an unsprung member of a suspension system to damp vibrations of the suspension spring, thereby improving the ride quality and the steering stability.
In vehicles having a relatively large load-carrying capacity, e.g. vans or station wagons, the vehicle height may vary to a considerable extent with a change in the carrying load due to the fact that the occupant gets on or off the vehicle, or goods or baggage is loaded onto or unloaded from the vehicle. This causes the ride quality and the steering stability to be degraded. Therefore, there has been a demand for a suspension system capable of automatically maintaining a constant vehicle height irrespective of variations in the carrying load.
Under these circumstances, there has heretofore been proposed a self-pumping hydraulic damper as disclosed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) No. 60-261713. In the conventional hydraulic damper, an oil tank and a reservoir have a high-pressure gas sealed therein. A pump device supplies a hydraulic fluid from the oil tank into a cylinder in response to the extension and contraction of a piston rod. A relief device relieves the pressure fluid in the pump device and the cylinder to the oil tank according to the position of the piston rod. The pressure in the cylinder is appropriately adjusted by operating the pump device and the relief device using vibrations of the suspension system during running of the vehicle so that the length to which the piston rod extends is adjusted to a constant value, thereby automatically maintaining a constant vehicle height.
However, the conventional self-pumping hydraulic damper suffers from some problems. After a vehicle equipped with the self-pumping hydraulic damper has run with no baggage loaded thereon, the pressure of the hydraulic fluid in the cylinder has been adjusted to a low level according to the small carrying load. If the vehicle under these conditions is loaded with baggage, the vehicle height lowers by an amount corresponding to an increase in the carrying load. As the vehicle loaded with the baggage runs, the pump device operates to supply the pressure fluid from the oil tank into the cylinder, causing the vehicle height to rise gradually. In this way, the vehicle height is adjusted to a predetermined level. In this case, when starting running where pumping has not yet sufficiently been effected, the vehicle runs in a state where the vehicle height is still low. Accordingly, the road clearance is insufficient, and the suspension system is likely to touch the ground. Therefore, when the carrying load is heavy or when the vehicle passes an uneven spot on the road, the suspension system may touch the ground unfavorably.